Conventional wire electrical discharge machining (EDM) equipment includes a head assembly mounted on a machine tool body and a numerically controlled positioning table. The wire equipment contains a spool assembly on which a fine wire is held taut between two spools. The drive spool moves the wire at a fixed rate from one spool to another with the useable work length between the two spools being about 4" to 8". The cutting rate depends upon the wire size and the amperage carried by the wire, plus the feed rate set for the depth of the cut. Deionized water is not only normally used as the dielectric, but also functions to flush out eroded particles produced during the cutting action. As is typical with all EDM operations, the wire electrode produces a uniform overcut or overburn which may be on the order of about 0.002" to 0.003" over the diameter of the electrode.
In the extrusion forming of ceramic honeycomb structures used in catalytic converters for internal combustion engines, it is necessary to form extrusion dies with substantially long, thin slots which are not only straight and extend across the face of the die, but which also have parallel sidewalls, are burr-free and have consistent surface finish. In addition, such long, thin slots should have a length greater than about 31/2", a depth in excess of about 0.06", and a width not greater than 0.012".
Known EDM wire processes, such as disclosed in U.S. Pat. Nos. 2,526,423; 4,205,213; and 4,233,486, are all capable of producing slots, however, such processes have a limited effective depth of slot which can be cut with a given wire diameter over a length of 31/2" or more within the surface of the workpiece. That is, for example, the difficulty encountered with known EDM wire processes of flushing out the eroded particles from within small slots during the cutting process, limits the effective depth of the cut. As the wire cutting process stalls, due to a buildup of eroded particles, the slot width becomes materially enlarged or unduly "overcut" at such depth, thus producing uneven slots.
Most EDM wire processes utilize a longitudinal or coaxial method of flushing wherein the deionized water is applied longitudinally along or coaxially of the wire making the longitudinal cut or slot in the workpiece. However, when machining such a longitudinal slot utilizing the wire process, the coaxially applied flushing media is deflected by the edge of the workpiece, and accordingly, the deflection of the dielectric often causes an inefficiency or breakdown in the cutting process. An inefficient machining condition is exhibited when the machine repeatedly backs away from the cutting surface and discontinues sparking, or when orange sparks are observed in the cutting area, or when persistent wire breakage occurs. As previously mentioned, such inefficient machining conditions not only seriously affect the ability to machine a slot to a given desired depth, but also "repeatability," which is the ability to consistently form identical slots, is virtually lost. Further, it has been found that a widening or bellmouth condition often occurs at the entrance of the slot adjacent the work surface when cutting with conventional EDM wire processes.
The present invention overcomes the problems encountered with known EDM wire processes and apparatus and permits the repeatable formation of a plurality of long, thin slots having parallel sidewalls with consistent surface finish and a depth in excess of 0.6" and a length of 31/2" or greater.